Background chloramphenicol resistance

[creeves]

I am hoping someone knows the cause of this phenomenon. Some E. coli strains derived from MM294 display weak resistance to chloramphenicol (34 mg/L). Specifically, if the strain is transformed with a low-copy, temperature-sensitive plasmid carrying the cat gene, colonies arise overnight at 30C. However, after incubation at 30C for 48 hours, smaller colonies of heterogeneous size arise (minute to just small). The same is observed if the untransformed host is plated at high density. In other words, the number of these small colonies is several orders of magnitude below the number plated. There is apparently weak resistance to chloramphenicol expressed stochastically in this strain. Has anyone also observed this and do you know the cause?